OSGB36

The Ordnance Survey National Grid reference system is a system of geographic grid references used in Great Britain, different from using Latitude and Longitude. It is often called British National Grid (BNG).

The Ordnance Survey (OS) devised the national grid reference system, and it is heavily used in their survey data, and in maps based on those surveys, whether published by the Ordnance Survey or commercial map producers. Grid references are also commonly quoted in other publications and data sources, such as guide books or government planning documents.

A number of different systems exist that can provide grid references for locations within the British Isles: this article describes the system created solely for Great Britain and its outlying islands (including the Isle of Man); the Irish grid reference system is a similar system created by the Ordnance Survey of Ireland for the island of Ireland. The Universal Transverse Mercator coordinate system (UTM) is used to provide grid references for worldwide locations and this is the system commonly used for the Channel Islands. European-wide agencies also use UTM when mapping locations, or may use the Military Grid Reference System (MGRS) system, or variants thereof.

The grid is based on the OSGB36 datum (Ordnance Survey Great Britain 1936, based on the Airy 1830 ellipsoid), and was introduced after the retriangulation of 1936–1962. It replaced the previously used Cassini Grid which, up to the end of World War Two, had been issued only to the military.

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